Stable free radical (SFR) mediated “living” polymerization has found use in various branches of polymer chemistry during the last decade, since it provides a new platform for controlling the free radical polymerization process1. A series of papers report on the synthesis of linear polymers of well defined molecular weights, the preparation of graft-2 or block-3 copolymers, and on polymerizations in emulsion4 or dispersion5. Most of these reports are exploiting the common features of SFR mediated polymerization systems, which are controlled rate of monomer incorporation into the growing polymer chain, a minimum of termination reactions compared to traditional free radical polymerization, and control of end group functionality or molecular shape and size.
It has recently been reported that porous poly(styrene-co-divinylbenzene) [poly(S-co-DVB)]monolithic polymers can be prepared in the presence of the stable nitroxide radical 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) by a mold polymerization process including the mono- and divinylic monomers as well as a porogenic solvent and a conventional free radical polymerization initiator6. The pore size distribution of these monolithic polymers differs fundamentally from the typical bimodal pore size distribution of poly(S-co-DVB) monoliths prepared without an SFR in the polymerization mixture7. For instance, the specific surface area was more than one order of magnitude higher (>300 m2/g) compared to monoliths prepared in “traditional” mold polymerizations without added TEMPO, at polymerization temperatures between 55 and 80° C. The pore size distribution was found to range from more than 1,000 nm to less than 10 nm, which rendered the monoliths, when used as a column packing material, more capable of separating molecular weight standards in size exclusion chromatographic (SEC) mode, compared to monolithic materials with a more pronounced bimodal pore size distribution. This porosity was attributed to be due mainly the high temperatures (>120° C.) required to accomplish the polymerization in the presence of TEMPO. The possibility to using the TEMPO radical, reversibly trapped during the mold polymerization, for carrying out grafting with different monomers on crushed monolith substrates has also been reported.
In spite of these promising characteristics, stable free radical (SFR) mediated polymerization of porous objects in closed molds yielded polymers with a high flow impedance. Good permeability at a relatively low back-pressure is essential for chromatographic and other flow applications, which made the monolithic columns prepared with TEMPO as SFR impractical. It is also essential to find faster polymerization systems requiring lower polymerization temperatures in order to broaden the class of monomers feasible for grafting, without excessive grafting due to thermally generated homopolymers8. This would cause a loss of SFR control, which is the primary advantage of SFR mediated polymerization.
Accordingly, there is a need for an improved polymerisation method in order to be able to produce monoliths that show improved flow permeability characteristics in size-exclusion chromatography.